Fingerprint sensing device with interposer structure

ABSTRACT

The invention relates to a fingerprint sensing device comprising: a sensing chip comprising an array of sensing elements being configured to be connected to readout circuitry for detecting a capacitive coupling between each of the sensing elements and a finger placed on a sensing surface of the sensing device. A surface of the sensing elements defines a sensing plane. The sensing device further comprises a plurality of interposer structures arranged on the sensing chip extending above sensing plane, wherein the plurality of interposer structures have the same height above the sensing plane. A protective plate is attached to the sensing chip by means of an adhesive, and the protective plate rests on the interposer structures such that a distance between the protective plate and the sensing plane is defined by the height of the interposer structures.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/168,591 filed on May 31, 2016, issued as U.S. Pat. No. 9,672,407,which claims the benefit of Swedish Patent Application No. 1550748-6filed Jun. 8, 2015, and Swedish Patent Application No. 1551288-2 filedOct. 7, 2015. The disclosures of the above applications are incorporatedherein by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates to a fingerprint sensing device. Inparticular, the present invention is related to a fingerprint sensingdevice comprising an interposer structure for enhancing the performancein the sensing device, and to a method for manufacturing a fingerprintsensor comprising such an interposer structure.

BACKGROUND OF THE INVENTION

As the development of biometric devices for identity verification, andin particular of fingerprint sensing devices, has lead to devices whichare made smaller, cheaper and more energy efficient, the possibleapplications for such devices are increasing.

In particular fingerprint sensing has been adopted more and more in, forexample, consumer electronic devices, due to small form factor,relatively beneficial cost/performance factor and high user acceptance.

Capacitive fingerprint sensing devices, built based on CMOS technologyfor providing the fingerprint sensing elements and auxiliary logiccircuitry, are increasingly popular as such sensing devices can be madeboth small and energy efficient while being able to identify afingerprint with high accuracy. Thereby, capacitive fingerprint sensorsare advantageously used for consumer electronics, such as portablecomputers, tablet computers and mobile phones, e.g. smartphones.

A fingerprint sensing chip typically comprises an array of capacitivesensing elements providing a measure indicative of the capacitancebetween several sensing structures and a finger placed on the surface ofthe fingerprint sensor. The sensing chip may further comprise logiccircuitry for handling addressing of the array of sensing elements.

A typical fingerprint sensor is protected so that the finger does notcome in physical contact with the sensing elements. In particular, itmay be desirable to arrange a glass plate on top of the sensor forprotecting the sensor, or to arrange the sensor behind a display glass.By arranging elements between the sensing surface and the sensingelements, the distance between the sensing surface and the sensingelements increases which reduces the capacitive coupling between afinger placed on a sensing surface of the device and the capacitivesensing elements.

With increased distance and reduced capacitive coupling, an increasingsensitivity is required of the sensing elements, i.e. the sensingelements must be able to measure a lower capacitance. With the sensingelements being pushed to the limit with regard to the minimum measurablecapacitance, it is increasingly important to ensure that the fingerprintsensor measures uniformly over the entire sensing area of the sensor.

In view of the above, it is desirable to improve the performance of afingerprint sensor having a low capacitive coupling between a fingerplaced on the sensing surface and the sensing elements.

Many attempts are made at improving the capacitive coupling, forexample, US2013/0201153 discloses a fingerprint sensing device whereelectrically conductive strands are arranged between the sensing surfaceand the sensing elements of a fingerprint sensing device. An insulatingmaterial is arranged between conductive strands. However, a directelectrical contact between the finger and the pixel may cause problemsrelated to electrostatic discharge (ESD). Moreover, the metallicportions of the surface may oxidize, resulting in undesirable aestheticeffects.

SUMMARY OF THE INVENTION

In view of above-mentioned desirable properties of a fingerprint sensingdevice, and drawbacks of prior art, it is an object of the presentinvention to provide a fingerprint sensing device and a method formanufacturing a fingerprint sensing device which improves capacitivemeasurements for capacitances near the limit of what is measureable.

According to a first aspect of the invention, there is provided afingerprint sensing device comprising: a sensing chip comprising anarray of sensing elements, the sensing elements being configured to beconnected to readout circuitry for detecting a capacitive couplingbetween each of the sensing elements and a finger placed on a sensingsurface of the sensing device, wherein a surface of the sensing elementsdefine a sensing plane; a plurality of interposer structures arranged onthe sensing chip extending above sensing plane, wherein the plurality ofinterposer structures have substantially the same height above thesensing plane; and a protective plate attached to the sensing chip bymeans of an adhesive arranged on the sensing chip, wherein theprotective plate rests on the interposer structures such that a distancebetween the protective plate and the sensing plane is defined by theheight of the interposer structures.

The sensing chip should in the present context be understood as a chipcomprising a plurality of sensing elements in the form of conductiveplates or pads, typically arranged in an array, which are capable offorming a capacitive coupling between each sensing element and a fingerplaced on an exterior surface of the fingerprint sensing device. Throughreadout of the capacitive coupling for each sensing element, ridges andvalleys of a fingerprint can be detected as a result of the distancedependence of the capacitive coupling. To achieve a fingerprint imagewith sufficient resolution, the sensing elements are typicallysubstantially smaller than the features (ridges and valleys) of thefinger. In general, a chip may also be referred to as a die.

The protective plate typically comprises a dielectric material in orderto provide a good capacitive coupling between a finger placed on theplate and the sensing elements of the sensing chip. In particular, theprotective plate may advantageously comprise a glass or ceramicmaterial, such as a chemically strengthened glass, ZrO₂ or sapphire. Theaforementioned materials all provide advantageous properties in thatthey are hard and thereby resistant to wear and tear, and in that theyare dielectric thereby providing a good capacitive coupling between afinger placed on the surface of the protective plate and the sensingelement of the sensing device. The protective plate described hereincommonly forms the outer surface of the fingerprint sensing device,hereinafter referred to as the sensing surface.

The sensing chip according to various embodiments of the invention maysubsequently be arranged on a conventional rigid PCB substrate or it maybe implemented using a flexible type of substrate comprising readoutcircuitry to form the fingerprint sensing device.

The present invention is based on the realization that a morehomogeneous distance distribution between the sensing surface and thesensing plane can improve the performance of a fingerprint sensingdevice. In particular, when a protective plate is used and the distancebetween the sensing surface and the sensing plane is increasing to thedegree that the difference in capacitance between a fingerprint ridgeand a valley is barely discernable by the sensing chip, it isincreasingly important that the capacitive coupling is as homogeneous aspossible over the entire surface of the sensing device.

Moreover, since a wafer coating material is sometimes provided on thesensing chip to protect and cover the sensing elements, it has beenrealized that the wafer coating material, if formed with a high degreeof accuracy, can be used as an interposer structure defining thedistance between the sensing plane and the sensing surface. Thethickness of the protective plate can be controlled to a high degree ofaccuracy, whereas the adhesive attaching the protective plate to thesensing chip typically is more difficult to deposit evenly, therebyexhibiting a more uneven surface. Thus, the distance between the sensingsurface and the sensing plane has previously been defined, at least inpart, by the adhesive attaching the protective plate, which may lead tosome inhomogeneity in the distance and in subsequent readout of afingerprint image.

According to embodiments of the present invention, interposer structuresexhibiting high thickness uniformity are provided on the sensingelements such that they have substantially the same height above thesensing plane. Accordingly, with the protective plate attached to thesensing chip by means of an adhesive, while resting on the interposerstructures, the distance between a finger placed on the protective plateand the sensing elements can be controlled with a high degree ofaccuracy. In such a situation, optimal settings can be applied acrossthe whole sensing area thereby facilitating acquisition of a fingerprintimage with high quality. Furthermore, the interposer structures arepreferably arranged and configured to provide sufficient mechanicalsupport for the protective plate to avoid movement or flexing of theprotective plate when the fingerprint sensor is in use.

The interposer structures can in principle be made from any commonlyused wafer coating material, which may refer to any material which isarranged to cover the sensing chip and in particular the sensingelements.

According to one embodiment of the invention, a variation in heightbetween the plurality of interposer structures may advantageously beless than 1 μm.

According to one embodiment of the invention, the plurality ofinterposer structures may comprise parallel lines, which would providean assembly of interposer structures which is simple to manufacture andwhich could provide sufficient mechanical support for the protectiveplate. The specific configuration of lines, such as the length, width,pitch and orientation can be selected based on the specificconfiguration of the fingerprint sensor. For example the parallel linesmay be aligned with an edge of the sensing chip and have a lengthsubstantially similar to the side of the sensing chip, thereby providinginterposer structures which define the distance between the sensingplane and the sensing surface over the full area of the sensing chip,including at the edges of the sensing chip to avoid that any edgeeffects occur.

Alternatively, according to one embodiment of the invention, theplurality of interposer structures may comprise one interposer structurecentered on each of said sensing elements. Thereby, it can be ensuredthat the distance from each individual sensing element to the sensingsurface is well defined and homogeneous over the full area of thesensing chip. It should be noted that many different configurations ofinterposer structures are possible, while still providing theadvantageous effects described above.

In one embodiment of the invention, in the case where one interposerstructure is located on each of said sensing elements, the interposerstructures may be formed in a material having a dielectric constanthigher than a dielectric constant of the adhesive. An improvedcapacitive coupling between a finger and a sensing element can beachieved by providing an interposer structure having a dielectricconstant which is higher than the surrounding adhesive. Thereby, theelectric field between a finger placed on the sensing surface and thesensing element can be focused towards the respective sensing elementsby means of the difference in dielectric constant. Accordingly, afurther improved fingerprint sensing device can be provided.

According to one embodiment of the invention, the plurality ofinterposer structures may be arranged in alignment with boundariesbetween the sensing elements, such that a central portion of eachsensing element is not covered by an interposer structure. For the aboveconfiguration of interposer elements, where at least a central portionof the sensing element is not covered by the interposer structure, theinterposer structures may be formed in a material having a dielectricconstant lower than a dielectric constant of the adhesive in order toachieve the above described effect of improved capacitive couplingbetween the sensing element and a finger. The adhesive will then fillthe space between interposer structures such that an inhomogeneous layeris formed with respect to the dielectric constant of the materials, andwhere the higher dielectric constant of the adhesive provides theimproved capacitive coupling.

According to one embodiment of the invention, the interposer structuresmay be arranged on the sensing chip at locations outside of a sensingarea of the sensing chip, the sensing area being defined by the array ofsensing elements. Thus, the interposer can be arranged to form a framepartially surrounding the array of sensing elements. The adhesive canthen be arranged primarily within the frame formed by the interposerelements

According to one embodiment of the invention, the plurality ofinterposer structures may comprise a photoresist. By using aphotoresist, the interposer structures can be formed using conventionalphotolithography and development processes, which simplifies the overallprocess flow. Moreover, a photoresist can easily be tailored to have aspecific dielectric constant so that a desired ratio of dielectricconstants can be achieved.

According to one embodiment of the invention, the sensing device mayfurther comprise a bond wire arranged between a bond pad on the sensingchip and a substrate on which the sensing chip is arranged, wherein abond loop height of the bond wire is lower than the interposerstructure. It may also be possible to have a bond wire loop height whichis slightly higher than the height of the interposer structure, in whichcase the protective plate may push down on the bond wire when theprotective plate is attached to the sensing chip.

In one embodiment of the invention, the sensing device may furthercomprise a via connection through said sensing chip to electricallyconnect the sensing chip to a substrate. A via connection, commonlyreferred to as a through-silicon via (TSV) connection, can be used toelectrically connect the sensing chip to readout circuitry on asubstrate without using wire bonding. TSV connection is for example usedwhen wire bonding is undesirable or unpractical, such as when thedesired height of the interposer structures is lower than a height ofthe wire bond. This situation could for example occur when the sensorshall be utilized with a very thick protective plate such as e.g. acover glass in a mobile phone.

According to a second aspect of the invention, there is provided amethod for manufacturing a fingerprint sensing device, the methodcomprising; providing a sensing chip comprising an array of sensingelements, the sensing elements being configured to be connected toreadout circuitry for detecting a capacitive coupling between each ofthe sensing elements and a finger placed on a sensing surface of thesensing device, wherein a surface of the sensing elements define asensing plane; forming an interposer layer on the sensing chip; forminga plurality of interposer structures from the interposer layer, whereinthe plurality of interposer structures have substantially the sameheight above the sensing plane; providing a liquid adhesive on thesensing chip; and arranging a protective plate on the sensing chip suchthat the adhesive is filling out spaces between the plurality ofinterposer structures, and such that the protective plate rests on theinterposer structures.

Through the above described manufacturing method, a fingerprint sensingdevice can be manufactured which exhibits the advantages described abovein connection with the first aspect of the invention.

That the adhesive is liquid should be interpreted to mean that it has afluidity which is sufficiently high to allow redistribution of theadhesive when the protective plate is arranged on the sensing chip. Theprotective plate is pressed down onto the sensing chip until it rests onthe interposer structures, thereby defining the distance between thesensing elements and the sensing surface. Through the redistribution ofthe adhesive it can be assured that voids between interposer structuresare filled and that superfluous adhesive is pushed out to the sides ofthe sensing chip.

According to one embodiment of the invention, the interposer layer maybe deposited by spin coating or by spray coating which are methods thatallow a high degree of accuracy and thickness uniformity when depositingthe interposer layer. Moreover, spin and spray coating representestablished methods which are compatible with conventional CMOSprocessing technologies, thereby providing a controllable manufacturingprocess which is easily integrated in existing manufacturing processes.

In one embodiment of the invention, providing the adhesive may comprisedispensing a liquid adhesive on and in between said interposerstructures. A liquid adhesive can easily be dispensed onto the sensingchip, over and in between the interposer structures. The uniformity whendispensing the adhesive is not crucial since the adhesive will beredistributed when the protective plate is pressed down onto theadhesive.

According to one embodiment of the invention, the interposer layer mayadvantageously comprise a photoresist, and the plurality of interposerstructures can thus be formed using photolithography, which is a wellestablished manufacturing technique that can be performed with highaccuracy, uniformity and repeatability.

According to one embodiment of the invention, the method may comprisethe steps of depositing a hard mask on the interposer layer, patterningthe hard mask, patterning the interposer layer according to the hardmask pattern, and removing the hard mask. Thereby, an alternativemanufacturing technique is provided. The hard mask can for example bemade from SiN and conventional photolithography may be used to form apattern in the hard mask which is subsequently transferred to theinterposer layer.

According to a third aspect of the invention, there is provided a methodfor manufacturing a fingerprint sensing device, comprising: providing asemiconductor wafer, on the semiconductor wafer forming a plurality offingerprint sensing chips, each sensing chip comprising an array ofsensing elements, the sensing elements being configured to be connectedto readout circuitry for detecting a capacitive coupling between each ofthe sensing elements and a finger placed on a sensing surface of thesensing device, wherein a surface of the sensing elements define asensing plane; forming an interposer layer on the semiconductor wafer,the interposer layer covering the plurality of fingerprint sensing chipsub-areas; forming a plurality of interposer structures from theinterposer layer, wherein the plurality of interposer structures havesubstantially the same height above the sensing plane; dicing thesemiconductor wafer into a plurality of individual sensing chips;arranging a sensing chip on a substrate comprising readout circuitry;electrically connecting the sensing elements of the sensing chip to thereadout circuitry; providing a liquid adhesive on the sensing chip; andarranging a protective plate on the sensing chip such that the adhesiveis filling out spaces between the plurality of interposer structures,and such that the protective plate rests on the interposer structures.

An advantage of the above described method is that an interposer layercan be deposited on a full wafer with a high uniformity usingestablished deposition techniques such as spin coating, therebyproviding a more effective manufacturing method where a large number ofsensing chips can be prepared simultaneously. Moreover, using spincoating or spray coating also allows the process to be easily modifiedwith respect to the desired thickness of the interposer layer.

Further features of, and advantages with, the present invention willbecome apparent when studying the appended claims and the followingdescription. The skilled person will realize that different features ofthe present invention may be combined to create embodiments other thanthose described in the following, without departing from the scope ofthe present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the present invention will now be describedin more detail, with reference to the appended drawings showing anexample embodiment of the invention, wherein:

FIG. 1 schematically illustrates a handheld electronic device comprisinga fingerprint sensing device according to an embodiment of theinvention;

FIGS. 2A-B schematically illustrate a fingerprint sensing deviceaccording to an embodiment of the invention;

FIGS. 3A-B schematically illustrate a fingerprint sensing deviceaccording to an embodiment of the invention;

FIGS. 4A-B schematically illustrates fingerprint sensing devicesaccording to embodiments of the invention;

FIG. 5 is a flow chart outlining the general steps of a method formanufacturing a fingerprint sensing device according to an embodiment ofthe invention;

FIGS. 6A-G schematically illustrate a method for manufacturing afingerprint sensing device according to an embodiment of the invention;and

FIG. 7 schematically illustrates a fingerprint sensing device accordingto an embodiment of the invention.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

In the present detailed description, various embodiments of afingerprint sensing device according to the present invention are mainlydiscussed with reference to a capacitive fingerprint sensing device. Amethod for manufacturing a fingerprint sensing device is also discussed.

FIG. 1 is a schematic illustration of a handheld device 100 comprising afingerprint sensing device 102 comprising a touchscreen display 104. Afingerprint sensing device 102 can be used in for example a mobilephone, a tablet computer, a portable computer or any other electronicdevice requiring a way to identify and/or authenticate a user.

FIG. 2A is a schematic illustration of a fingerprint sensing device 200according to an embodiment of the invention, as seen in a top view. Inparticular, FIG. 2A illustrates the outline of a plurality of interposerstructures 202, where each interposer structure 202 is arranged on asensing element 204 of the sensing device 200.

FIG. 2B illustrates the sensing device 200 in further detail where asensing chip 206 comprising an array of sensing elements 204 is shown.Here it can be seen that an adhesive 208 is arranged in the spacebetween interposer structures 202, and that a protective plate 210 isattached to the sensing device 200 by means of the adhesive 208. Theprotective plate 210 can be manufactured with a high degree of accuracy,and the variation in thickness over the sensing area is typically lessthan 2 μm. Moreover, the distance between the surface of the sensingelements 204 and the surface 212 of the protective plate 208, is definedby the height of the interposer structures 202. The height of theinterposer structures is typically in the range of 5 to 50 μm. Theheight the of the interposer structures for a specific application canfor example be selected based on the bonding technique used to connectthe sensing chip to a substrate, or on the type of adhesive used. Thesurface plane of the sensing elements 204 is defined as the sensingplane, and the surface 212 of the protective plate 210 is defined as thesensing surface 212.

The protective plate 210 may also be the cover glass in a devicecomprising a touch screen, and a cover glass covering the fingerprintsensing device may also be covering the display and touchscreen portionsof the handheld device. In principle, the protective plate may be anystructure which acts to cover and protect the sensing device while stillallowing a capacitive coupling between a finger placed on the surface ofthe protective plate and the sensing elements.

The sensing elements 204 are here shown arranged in a square array, thesensing elements having a size of about 50×50 μm and a distance betweenadjacent elements is about 5 μm. The sensing elements 204 areelectrically conductive, typically metallic, and can as a generalapproximation be considered to act as one plate in a parallel platecapacitor, where a finger placed on a sensing surface 212 of thefingerprint sensing device 200 represents the other plate. Each sensingelement 204 is connected to readout circuitry (not shown) for detectinga capacitive coupling between each of said sensing elements 204 and afinger placed on the sensing surface 212.

FIG. 3A is a schematic illustration of a fingerprint sensing device 300according to an embodiment of the invention where the interposerstructures 302 are provided in the form of parallel lines, or ridges,aligned with the boundaries between sensing elements 204.

In FIG. 3B it can be seen that the adhesive is arranged to fill out thespace between the ridges 302 and to attach the protective plate 210 tothe sensing chip 206.

FIG. 4A is schematic illustration of an alternative embodiment of afingerprint sensing device 400 where interposer structures 402 arearranged aligned with boundaries between adjacent sensing elements 204but with a gap or opening 306 in the interposer structures 402 betweenadjacent interposer structures 402.

FIG. 4B is schematic illustration of an alternative embodiment of afingerprint sensing device 410 where interposer structures 412 arearranged on the sensing chip outside of the array of sensing elements204, which defines the sensing area. Thereby, the adhesive 208 can bearranged to completely cover the sensing elements 204, providing ahomogeneous covering layer. As can be seen in FIG. 4B, there is a spacebetween adjacent interposer structures 412 enabling a liquid adhesive toflow out through the gaps formed between the structures when theadhesive is deposited. Thereby, any additional adhesive can be pushedout outside of the interposer structures 412 when the protective plate210 is arranged on the sensing device 410 so that the protective plate210 rests on the interposer structures 412. It should be noted that theconfiguration illustrated in FIG. 4B is one of many possibleconfigurations for the interposer structures, and that many differentconfigurations of the interposer elements are possible, where the mainfeature is that the interposer structures are all of the same height andthat they provide mechanical support for the protective plate.

An advantage of providing interposer structures as illustrated in FIGS.2-4 above is that when a liquid adhesive is being dispensed, theadhesive can easily flow out and fill all the spaces between interposerstructures, thereby forming a homogeneous layer without air gaps, whichin turn leads to a well defined dielectric structure between the sensingelements 204 and the sensing surface 212. The adhesive may for examplehave the same dielectric constant as the material from which theinterposer structures are made, thereby making the layer comprising theinterposer structures and the adhesive behave as a uniform layer in adielectric perspective.

Alternatively, it is possible to use an adhesive and an interposerstructure having different dielectric constants, in which case thedifference in dielectric constant can be used to focus the electricfield towards the sensing elements. This requires that the materiallocated directly above each sensing element should have a higherdielectric constant than the surrounding material. Taking the sensingdevice 200 of FIGS. 2A-B as an example, the interposer structures 202would need to have a dielectric constant which is higher than thedielectric constant of the adhesive 208 to achieve the field focusingeffect.

FIG. 5 is a flow chart outlining the general steps of a manufacturingmethod according to an embodiment of the invention. The manufacturingmethod will be discussed also with reference to FIGS. 6A-G.

First, in step 502 illustrated in FIG. 6A, a circular wafer 600comprising a plurality of sensing chips 602 is provided. The wafer 600may for example be a silicon wafer where sensing chips 602 have beenformed using conventional CMOS-compatible processing. By using a fullsize circular wafer, large scale processing advantages can be achieved.

Next 504, as illustrated in FIG. 6B, an interposer layer 604 is formedon the surface of the wafer 600, thereby covering the sensing chips 602to form a uniform layer on the surface of the wafer 600. The interposerlayer 604, which may also be referred to as a coating layer, is formedto have a uniform thickness and to cover the entire area of the wafer600. The interposer layer 604 can for example be a photoresist depositedby spin-coating or spray coating, and the photoresist may be either apositive or a negative photoresist. Spin- and spray-coating are wellestablished manufacturing techniques which can be performed with highaccuracy on wafer scale, thereby providing an interposer layer 604having a uniform thickness over the surface of the wafer 600. To achievea high homogeneity in the deposited layer, spray coating canadvantageously be performed using ultrasonic nozzles. Furthermore, it isalso possible to use methods such as inkjet printing or 3D-printing toform the interposer structures.

In the next step 506, illustrated in FIG. 6C, interposer structures 606are formed on the surface of the wafer using photolithography. Here, theinterposer structures are provided in the form of individual structureswhere each structure 606 is centered on a corresponding sensing element608. The cross section of the interposer structure 606 is illustrated asbeing substantially hexagonal. However, the cross section of theinterposer structure may in principle be selected arbitrarily, and itmay be circular, quadratic or have any polygonal shape there between.Moreover, it is not required that the interposer structure is symmetric,nor does it need to be symmetric in the vertical direction.

As an alternative to using a photoresist to form the interposerstructures as described above, it is also possible to form theinterposer structures in another material. As an example, a hard maskmay be formed on the wafer, for example a SiN mask, after which the hardmask is patterned using photolithography, patterning and subsequent deepreactive ion etching (DRIE). It is also possible to use laser ablationsto remove material in selected areas to form the desired patterns ofinterposer structures. Moreover, it is also possible to use additivetechniques for the purpose of fabricating the interposer structures andassociated geometries, such as e.g. inkjet printing or 3D printing.

After forming the interposer structures, the interposer layer may betreated in a plasma cleaning process in order to improve adhesionbetween the interposer structures and the subsequently depositedadhesive. The plasma cleaning may for example comprise oxygen mixed withan inert gas such as nitrogen or argon.

After forming the interposer structures 606 on the wafer, the wafer isdiced 508 into separate sensing chips 602 illustrated in FIG. 6D. FIG.6D further illustrates a bond pad 609 on the sensing chip 602subsequently used to electrically connect the sensing chip 602 to asubstrate. As illustrated in FIG. 6E, the individual sensing chips 602are subsequently arranged 510 onto corresponding substrates 612comprising readout circuitry (not shown) for reading out the informationfrom the sensing elements 608 of the sensing chip 602 in order to form afingerprint image. The sensing chip 602 is connected 512 to thesubstrate 612 by means of a bond wire 614 reaching from a first bond pad616 on the sensing chip to a second bond pad 618 on the substrate. Here,only one bond wire is illustrated to avoid cluttering the drawings.

As a next step 514, illustrated in FIG. 6F, a liquid adhesive 620 isprovided 514 by dispensing the adhesive 620 onto the interposer layer sothat the adhesive 208 fills the spaces between the interposer structures606.

In the final step 516 as illustrated in FIG. 6G, a protective plate 210is attached to the sensing device by means of the adhesive 208. Theprotective plate 210 is arranged onto the adhesive 620 and a certainpressure is applied so that the adhesive 620 is redistributed to fillall the spaces between adjacent interposer structures 606. Theprotective plate is pressed down until it rests on the interposerstructures. After the step of applying the adhesive on the sensing chip,there could be a drying step involved (sometimes referred to as betastage curing) to partially dry the adhesive. In case of curing, theprotective plate can be attached to the partially cured/dried adhesivein a subsequent assembly step by applying heat and pressure.

FIG. 7 illustrates a sensing device where the electrical connectionbetween the sensing elements and the substrate 612 is formed using a viaconnection 702 through the sensing chip 602. Such a via connection 702may also be referred to as a through silicon via (TSV) connection.

Even though the above method is illustrated as starting from a fullwafer, it is equally possible to form the interposer structures on analready diced sensing chip.

Moreover, the protective plate may also be provided with a frame, whichmay also be referred to as a bezel, surrounding the sensing chip oncethe protective plate is in place. The bezel may for example help toprotect the bond wires between the sensing chip and the substrate. Thebezel may also be a conductive structure acting as a drive element for afinger, and/or functioning as an ESD discharge node.

It should be noted that the general aspects of the invention discussedherein are not limited to the specific dimensions and sizes disclosed inthe present description. The above description merely provides anexample embodiment of the inventive concepts as defined by the claims.

Even though the invention has been described with reference to specificexemplifying embodiments thereof, many different alterations,modifications and the like will become apparent for those skilled in theart. Also, it should be noted that parts of the device and method may beomitted, interchanged or arranged in various ways, the device and methodyet being able to perform the functionality of the present invention.

Additionally, variations to the disclosed embodiments can be understoodand effected by the skilled person in practicing the claimed invention,from a study of the drawings, the disclosure, and the appended claims.In the claims, the word “comprising” does not exclude other elements orsteps, and the indefinite article “a” or “an” does not exclude aplurality. The mere fact that certain measures are recited in mutuallydifferent dependent claims does not indicate that a combination of thesemeasures cannot be used to advantage.

The invention claimed is:
 1. A fingerprint sensing device comprising: asensing chip comprising an array of sensing elements, said sensingelements being configured to be connected to readout circuitry fordetecting a capacitive coupling between each of said sensing elementsand a finger placed on a sensing surface of said sensing device, whereina surface of said sensing elements define a sensing plane; a pluralityof interposer structures arranged on said sensing chip extending abovesaid sensing plane, wherein said plurality of interposer structures havethe same height above said sensing plane; and a protective plateattached to said sensing chip by means of an adhesive arranged on saidsensing chip, wherein said protective plate rests on said interposerstructures such that a distance between said protective plate and saidsensing plane is defined by the height of said interposer structures. 2.The sensing device according to claim 1, wherein a variation in heightbetween said plurality of interposer structures is less than 1 μm. 3.The sensing device according to claim 1, wherein said plurality ofinterposer structures comprises parallel lines.
 4. The sensing deviceaccording to claim 3, wherein said parallel lines are aligned with anedge of said sensing chip and have a length substantially similar to aside of the sensing chip.
 5. The sensing device according to claim 1,wherein said plurality of interposer structures comprises one interposerstructure centered on each of said sensing elements.
 6. The sensingdevice according to claim 5, wherein said plurality of interposerstructures are formed in a material having a dielectric constant higherthan a dielectric constant of said adhesive.
 7. The sensing deviceaccording to any one of claim 1, wherein said plurality of interposerstructures are arranged in alignment with boundaries between saidsensing elements, such that a central portion of each sensing element isnot covered by an interposer structure.
 8. The sensing device accordingto claim 7, wherein said plurality of interposer structures are formedin a material having a dielectric constant lower than a dielectricconstant of said adhesive.
 9. The sensing device according to claim 1,wherein said interposer structures are arranged on said sensing chip atlocations outside of a sensing area of said sensing chip, said sensingarea being defined by said array of sensing elements.
 10. The sensingdevice according to claim 1, wherein said interposer structures arearranged to form a frame partially surrounding the array of sensingelements.
 11. The sensing device according to claim 10, furthercomprising a space between adjacent interposer structures of said frameenabling a liquid adhesive to flow out through gaps formed between thestructures when said liquid adhesive is deposited.
 12. The sensingdevice according to claim 10, wherein the adhesive is arranged withinthe frame formed by said interposer.
 13. The sensing device according toclaim 10, wherein the adhesive is arranged to completely cover the arrayof sensing elements.
 14. The sensing device according to claim 1,wherein said plurality of interposer structures comprises a photoresist.15. The sensing device according to claim 1, further comprising a bondwire arranged between a bond pad on said sensing chip and a substrate onwhich the sensing chip is arranged, wherein a bond loop height of saidbond wire is lower than said interposer structure.
 16. The sensingdevice according to claim 1, further comprising a via connection throughsaid sensing chip to electrically connect said sensing chip to asubstrate.
 17. An electronic device comprising a sensing deviceaccording to claim 1.